Email to Yahoo being delayed 4 to 12 hours

While experimenting with setting up hosted email with Zoho, so that several owned domains will have email sent to/from my Zoho account, I experienced the following:

Emails sent from my Zoho account (under various aliases) to my Yahoo email acct are experiencing defer/delays at Yahoo from 4 to 12 hours. The first 3 or 4 emails sent to the Yahoo account are received quickly, but thereafter, the defer/delay occurs.

Each deferred/delayed email eventually is delivered to Yahoo, and my Zoho account receives a 421 message from Yahoo (saying that the email was deferred/delayed).

I tried the same experiment using Google Apps as an email host, sending the same type of test emails from aliases in my Google Apps account into Yahoo. All of these emails were received on time, and none produced the Yahoo delays or 421 message.

I conclude that Yahoo is deferring/delaying zoho mail in particular (as compared to not delaying the same type of email from the Google Apps account).

Is Zoho aware of this? If so, it is really not enough of an answer (as Zoho answered a few years ago to a similar question) that it is "Yahoo's occasional delaying of mail", since Yahoo is not delaying the same type of email coming from Google Apps.

I sent a Tech Support message to Zoho, which included the email delivered from Zoho to Yahoo, the header, and the 421 message received in my Zoho account. After several days, I have not received any sort of response from Zoho tech support.

Can Zoho shed any light on what is occurring?
Thanks. --Bill